As is further discussed below, the invention is not limited in application to oil sand; however it has been developed in connection with the treatment of such material and therefore the following disclosure describes it in connection with that particular feedstock.
Whole oil sand is a material whose composition and characteristics have direct influence on the design of apparatus and method for treating it. Its composition includes granular mineral solids, water and bitumen. The granular solids comprise coarse solids (mainly sand having particle sizes in the range 80-200 mesh) and fine solids (mainly clay having particle sizes less than about -44.mu.). The whole oil sand further comprises rocks, ranging from pebbles to boulders, and cohesive lumps of granular solids. In winter, the whole oil sand, which is mined by huge draglines or rotating bucket wheels, commonly reports in the form of frozen chunks--some weighing in the order of two tons. In summer, the as-mined material reports as a sticky mass which is difficult to screen to remove the oversize rocks and lumps. When used herein, the term "whole oil sand" means this as-mined material, although it may have been subjected to preliminary rough screening or the like to remove easily separable large boulders.
Oil sands are today commercially treated with what is commonly known as the hot water extraction process. This process involves first conditioning the whole oil sand by mixing it with steam and some hot water for a period of time in a horizontal rotating drum. With heat and dilution, the solid and hydrocarbon components of the oil sand separate to an extent which permits oversize material to be removed by screening. The product is then diluted with hot water and introduced into a settling tank. Here the coarse sand settles out and is discarded as an underflow. The bitumen, attached to air bubbles incorporated in the mixture in the conditioning drum, floats as a froth and is recovered. A dragstream--containing mainly water, some non-floatable bitumen, and fine solids--is drawn from the centre of the tank. This dragstream is treated in a sub-aerated flotation cell to produce a contaminated bitumen froth and a watery underflow. The underflows are combined and discarded; the froths are combined, cleaned to remove containined water and solids, and then upgraded in a conventional refinery on--site operation.
There are presently two commercial plants of this type in operation in Canada. The second-built plant is designed to produce about 125,000 barrels of net synthetic crude per day and its construction cost, including the mining and upgrading facilities, was in the order of several billions of dollars.
There are a number of disadvantageous features, of interest with respect to this invention, which characterize the hot water extraction process. For example, it uses enormous quantities of water. Since it is intended to produce a million or more barrels of product per day from the Canadian oil sands, the pressure on the finite water supplies in the oil sand area is a serious problem. Secondly, the wet tailings produced have to be retained for years in gigantic diked ponds before the water in them is sufficiently clean to be re-used in the process. This is because the clay particles suspended in the water are very slow to settle out. Thirdly, the need to heat process water and produce steam consumes some of the hydrocarbons produced. Fourthly, there is a need to upgrade the bitumen with an on-site refinery before it can be pumped, due to its high viscosity.
With these disadvantages in mind, it has heretofore been proposed to pyrolyze the oil sand using a solid carrier to provide the heat. More particularly, this "dry" scheme contemplates mixing oil sand with hot recycled sand, thereby effecting heat transfer and vaporizing and cracking bitumen and producing coked sand. The coke on the coked sand is subsequently burned to heat the sand so that it may be recycled to the heat transfer operation.
This dry scheme reduces water consumption and disposal. It has the possibility of yielding higher liquid hydrocarbon recoveries than the hot water process including refining. Furthermore, it will yield a less viscous liquid product which will be more easily pumpable than the hot water process product.
This invention is concerned with a novel dry processor and with the process performed in it.
The present processor has been developed with the following criteria in mind. It should be capable of doing the following:
1. processing whole oil sand without or with minimal prior screening;
2. converting whole oil sand into a form from which oversize rocks may be separated and rejected and then making such a separation;
3. reducing the size of lumps of oil sand so that at least some of them become part of the normally processable feed stream;
4. heating the material in stages so as to vaporize the water and hydrocarbons in different zones, with the result that they may be separately recovered and thus do not contaminate each other to an undesirable extent;
5. vaporizing and cracking hydrocarbons so that they may be withdrawn and collected in a desirable product form;
6. conserving energy by burning coked sand to provide some, if not all, of the heat needed for the process in the form of hot sand, from which heat may be extracted by heat transfer;
7. efficiently recovering heat from recycled hot sand to further conserve energy; and
8. carrying out these operations in a single processor unit which is capable of maintaining substantial segregation of the gaseous atmospheres in the various zones where different operations are simultaneously processing.
Bennett, in U.S. Pat. No. 3,481,720, describes a dry thermal processor which meets some, but not all, of these objectives. This processor was developed in connection with treating oil shale, but the patent states it has utility for oil sand as well. The Bennett unit comprises rotatable, horizontal, concentrically arranged, spaced inner and outer tubes having first and second ends. The inner tube provides a preheat zone at its first end and a vaporization zone at its second end. The annular space between the tubes is divided into a combustion zone at the second end and a heat-transfer zone at the first end. The feed is pre-crushed and then advanced through the pre-heat, vaporization, combustion and heat-transfer zones sequentially and undergoes different processes as described below. In general, the method embodied in the operation of the unit comprises:
(a) feeding crushed raw feed stock into the pre-heat zone. Here the feed is heated to about 200.degree.-300.degree. F. by heat exchange, through the inner tube wall, with hot gases associated with hot solids advancing through the heat-transfer zone of the annular space. By preheating the raw feed, contained moisture is converted into steam, which is recovered through a pipe extending into the preheat zone;
(b) Mixing the preheated feed with hot recycled solids in the vaporization zone and effecting heat exchange through solids contact to produce a mixture having a temperature in the order of 900.degree. F. As a result of this operation, contained hydrocarbons are volatilized, some are cracked, and the gaseous products are recovered from the zone through a pipe extending thereinto--also, coked solids are left as a residue;
(c) Transferring the coked solids into the combustion zone and mixing them with injected air to effect combustion of the coke and raise the temperature of the solids to 1400.degree.-1600.degree. F.;
(d) Recycling a portion of the hot solids produced in the combustion zone into the vaporization zone to heat the preheated solids and advancing the balance of the hot solids past the recycle point and through the heat-exchange zone, to heat the wall of the inner tube and thus the solids contained therein in the preheat zone; and
(e) Discharging the solids from the first end of the heat-exchange zone to waste.
Bennett teaches the use of augers to advance the solids through the inner tube and back through the annular space. The augers are welded around their inner and outer circumferences to the relevant containing tube. By a combination of these augers and a choking action using feed solids (achieved by varying the pitch of the augers), Bennett segregates the preheat zone gases, the vaporization zone gases and the combustion zone gases from each other.
With respect to the objectives previously set forth, it will be noted that Bennett relies on:
(1) Augers to move the material. This requires that the feed stock be of generally uniform particle size. Bennett achieves this by requiring crushing of the feedstock before it is introduced into the processor;
(2) The augers and solids choking to achieve segregation of the gaseous atmospheres; and
(3) The use of gases in the heat-transfer zone to conduct heat from the recycled hot solids in the zone to the wall of the inner tube for conductance through the wall to the solids in the pre-heat zone.